In a saucepan over medium heat, combine the 400g chopped rhubarb, 200g sugar, and a pinch of salt.

Cook for about 10-15 minutes until the rhubarb has broken down into a thick compote. Stir occasionally.

Remove from heat and let it cool while you prepare the custard.

In a bowl, whisk together 55g sugar, 23g custard powder, and 60g egg yolks until smooth.

In a saucepan, heat 205g whole milk and ½ tsp vanilla bean paste until just simmering.

Gradually whisk the hot milk into the egg mixture, then return it all to the saucepan.

Cook over low heat, stirring constantly, until the mixture thickens (about 5-7 minutes). Be careful not to let it boil.

Once thickened, stir in 10g unsalted butter and let cool.

In a mixing bowl, combine 290g all-purpose flour, 30g sugar, 1 tsp yeast, and 1/2 tsp salt.

Add 120g whole milk, 1 tsp vanilla, and the whisked egg. Mix until a shaggy dough forms.

Knead in 60g room temperature butter until smooth, about 5-7 minutes. The dough should be soft and elastic.

Let it rise in a greased bowl, covered, for about 1 hour or until doubled in size.

In a bowl, mix 25g turbinado sugar, 25g brown sugar, 65g flour, ½ tsp cinnamon, and a pinch of salt.

Add the 35g melted butter and mix until crumbly. Set it aside.

Preheat your oven to 180°C (350°F).

Roll out the dough on a lightly floured surface into a rectangle, about 1 cm thick.

Cut the dough into squares, about 10x10 cm. Fill the center of each with a tablespoon of rhubarb compote and custard filling.

Fold the corners of each square to the center, creating a bun shape. Place them seam-side down on a parchment-lined baking sheet.

Sprinkle streusel topping generously over the buns.

Let them rise for 30 minutes, then bake for 20-25 minutes or until golden brown.